A traditional hollow fiber membrane module for gas separation typically has a feed gas inlet at one end, a retentate gas outlet at an opposite end and a permeate gas outlet therebetween and along a side of the module. The feed gas inlet and retentate gas outlet are thus separated from one another by the full length of the module, providing a long distance for the feed gas to travel from the inlet to the outlet.
Heretofore, most gas separation modules have been designed to provide co-current or counter-current gas flow patterns through their hollow fiber membrane core in order to maximize the separation of the fast permeate portion of the feed gas and the high concentration thereof in the permeate gas. Such gas flow patterns through the hollow fiber membrane core may produce high concentrations of a constituent permeate gas, for example, from 95% to 99.99% N.sub.2 or O.sub.2.
The need for a suitable O.sub.2 /N.sub.2 selective, high permeability, hollow fiber membrane gas separation module for use within a diesel automobile, bus or other combustion vehicles, has recently been emphasized by Geoff R. Rigby and Harry C. Watson in a paper entitled "Application of membrane gas separation to oxygen enrichment of diesel engines" appearing in Journal of Membrane Science, 87 (1994) 159-169. However, as recognized by the inventors herein, before a suitable hollow fiber membrane gas separation module adapted for O.sub.2 enrichment can be practically applied, several important criteria will have to be met. The gas separation module must be inexpensive to produce and easy to service and/or replace due to fouling, have a high permeate flow rate, work effectively under relatively low pressures, and be able to fit within a small volume. Previous designs have met some but not all of these important criteria.
One possible gas separation module design is disclosed in U.S. Pat. No. 5,174,900 to Nichols et al. This module has an outer pressure vessel and membrane wafers placed inside the pressure vessel. The membrane wafers are in the form of mats composed of hollow fibers woven into a textile. These membrane wafers are stacked together to a suitable height in the pressure vessel and sealed around their outside edge, making circular disks. The gas separation module is fed transversely, allowing high flow rates.
This gas separation module design meets many of the important criteria for a good oxygen enrichment membrane module. For example, the membrane wafers in the module vessel may be removed and replaced. However, overall, this gas separation module design is expensive and complex and not suitable for low cost construction.
Another possible gas separation module design is disclosed in U.S. Pat. No. 5,051,113 to Nemser. This gas separation module provides O.sub.2 enriched air for an internal combustion engine. The module has an outer pressure vessel filled with axially extending hollow fiber membranes each having a high permeability selective layer to separate oxygen from air.
While this gas separation module design appears to be simple and easy to produce, it would occupy a volume much too large to fit into the engine compartment of a vehicle. In addition, the gas separation module is not designed to run under low pressures, such as less than 100 psi. Lastly, no way is provided to service the membrane or to replace it should it become fouled.
Thus, a need exists for improvements to meet the aforementioned important criteria in gas separation module design, particularly for combustion applications.